A head-mounted display (HMD) device, through an optical system, augments images from an image source and presents the images in front of a user's eyes, displaying different visual effects according to different applications, such as 3D display, augmented reality application, etc. Because the images typically only enter the user's eyes of his/her own, the head-mounted display device, compared with conventional displays, has better privacy. As technologies of virtual reality (VR) and augmented reality (AR) develop rapidly, HMDs are receiving more and more attention because of their huge market values and development potentials.